All old editions of D&D derive to-hit directly from HD (although the 2E manuals print THAC0 separately and often actually get it wrong), so that’s not problem at all. You’ll just have to re-roll hit points for PCs. Monsters, though, require next to no conversion in my experience: you just adjust AC by 1 and recalculate XP values. (I’ve converted over a pile of AD&D 2E monsters so far.)
AD&D monsters improve their to-hit by 2 per 2 HD. ACKS does to-hit improvements much more sensibly than AD&D, IMO; instead of giving monsters a bump of 2 every 2 HD, they get 1 per 1 HD. The same for clerics. I really see no good reason to not increase THAC0 at 1:1 for monsters in AD&D 2E (although I have a vague recollection of having read a decent one, I still didn’t agree with it).
3E, though… forget about it, really. You’d have to build everything from scratch. Even HD don’t really translate over directly, IMO, because the “value” of 3E HD is so different.
I think the attack throw thing is that there’s some “offset” between B/X and AD&D attacks. B/X starts descending AC at 9 (chain mail at 5), and you need a 10+ on d20 to hit AC 9 (14+ to hit chain mail), by default. ACKS starts ascending AC at 0 (chain mail at 4), and you need a 10+ to hit AC 0 (14+ to hit chain mail).
However, in AD&D, AC starts at 10, chain mail is AC 5, and then things get complicated…
In 1E, by the matrixes, a 1st-level character (other than magic-users and illusionists) needs an 10+ to hit AC 10. That means 15+ to hit AC 5! That’s 5% harder than in B/X or ACKS. A 0-level man or a 1st-level magic-user/illusionist needs 11+ to hit AC 10, or 16+ to hit AC 5. (10% harder!) A 1 HD monster needs 9+ to hit AC 10, and 14+ to hit AC 5. HD 1-1 monsters need 10+ for AC 10, and monsters with less than 1-1 HD need 11+. Monsters with 1+1 HD need 8+, and then we get a staggered progression…
In 2E, things change again! 0-level people to 1st-level PCs need 10+ to hit AC 10, and 15+ to hit AC 5 (so, flat 5% harder than in B/X or ACKS). Monsters of 1 HD need 9+ to hit AC 10, and 14+ to hit AC 5. 2E also switches fighters from a “2:2” progression to a “1:1” progression, and gives monsters a “2:2” progression (so their THAC0 is always odd past 1 HD; 17 at 3 HD, 15 at 5 HD, but never 18 or 16).
So the to-hit numbers are basically off by 5% between AD&D and ACKS (and also, 1E is kind of crazy).